A transfer machine uses a different machine tool at each individual station. Workpieces are caused to pass through these stations to undergo a series of machining processes. In recent years, a flexible transfer machine has been put into practical use to perform machining operations flexibly. Specifically, this machine makes use of numerically controlled machine tools disposed at various stations. Usually, each individual numerically controlled machine tool is loaded with a program to be executed at the corresponding station and, therefore, each machine tool simply executes that program.
If a trouble occurs at some station, then the machining process assigned to the station is not performed. Therefore, the machining process cannot be continued until the whole transfer machine is stopped and the station in trouble is repaired. Especially, in a system using a number of numerically controlled machine tools, troubles often take place. Each time a trouble occurs at some station, the whole system must be stopped. Hence, the machining efficiency is low.